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The boundaries of the West African craton, with special reference to the basement of the Moroccan metacratonic Anti-Atlas belt

¹ Geodynamic Laboratory, El Jadida University, BP. 20, 24000, El Jadida, Morocco (e-mail: ennih{at}ucd.ac.ma)
² Isotope Geology, Royal Museum for Central Africa, B-3080 Tervuren, Belgium (e-mail: jean-paul.liegeois{at}africamuseum.be)

	Abstract

Top Abstract The West African craton The boundaries of the... The case study of... Conclusions References

 
The West African craton (WAC) was constructed during the Archaean and the c. 2 Ga Palaeoproterozoic Eburnian orogeny. Mesoproterozoic quiescence at c. 1.7–1.0 Ga allowed cratonization. In the absence of Mesoproterozoic activity, there are no known WAC palaeogeographical positions for that time. At the beginning of the Neoproterozoic, the WAC was affected by several extensional events suggesting that it was subjected to continental breakup. The most important event is the formation of the Gourma aulacogen in Mali, and the Taoudeni cratonic subcircular basin and deposition of platform sediments in the Anti-Atlas. At the end of the Neoproterozoic, the WAC was subjected to convergence on all its boundaries, from the north in the Anti-Atlas, to the east along the Trans-Saharan belt, to the south along the Rockelides and the Bassarides and to the east along the Mauritanides. This led to a partial remobilization of its cratonic boundaries giving rise to a metacratonic evolution. The WAC boundaries experienced Pan-African Neoproterozoic to Early Cambrian transpression and transtension, intrusion of granitoids and extrusion of huge volcanic sequences in such as in the Anti-Atlas (Ouarzazate Supergroup). Pan-African tectonism generated large sediment influxes around the WAC within the Peri-Gondwanan terranes whose sedimentary sequences are marked by distinctive zircon ages of 1.8–2.2 Ga and 0.55–0.75 Ga.

WAC rocks experienced Pan-African low grade metamorphism and large movements of mineralizing fluids. In the Anti-Atlas, this Pan-African metacratonic evolution led to remobilization of REE in the Eburnian granitoids due to the activity of F-rich fluids linked to extrusion of the Ouarzazate Supergroup. During the Phanerozoic, the western WAC boundary was subjected to the Variscan orogeny, for which it constituted the foreland and was, therefore moderately affected, showing typical thick-skin tectonics in the basement and thin-skin tectonics in the cover. During the Mesozoic, the eastern and southern boundaries of the WAC were subjected to the Atlantic opening including Jurassic dolerite intrusion and capture of its extreme southern tip by South America. The Jurassic is also marked by the development of rifts on its eastern and northern sides (future Atlas belt). Finally, the Cenozoic period was marked by the convergence of the African and European continents, generating the High Atlas range and Cenozoic volcanism encircling the northern part of the WAC. The northern metacratonic boundary of the WAC is currently uplifted, forming the Anti-Atlas Mountains.

The boundaries of the WAC, metacratonized during the Pan-African orogeny have been periodically rejuvenated. This is a defining characteristic of the metacratonic areas: rigid, stable cratonic regions that can be periodically cut by faults and affected by magmatism and hydrothermal alteration – making these areas important for mineralization.

This special volume has been generated by the UNESCO IGCP485 (International Geological Correlation Programme, now International Geoscience Programme) called Cratons, metacratons and mobile belts: keys from the West African craton boundaries; Eburnian versus Pan-African signature, magmatic, tectonic and metallogenic implications. The aim of this project, and of this book, was to encompass the whole evolution of the boundaries of the West African craton, from the Archaean/Palaeoproterozoic towards Recent times. The IGCP485 organized field conferences in remote areas such as the Reguibat Rise in Mauritania, the Gourma region in Mali, the Hoggar shield in Algeria and twice in the Anti-Atlas belt in Morocco. This book contains twenty-four papers concerning these regions and other boundaries of the West African craton.

	The West African craton

Top Abstract The West African craton The boundaries of the... The case study of... Conclusions References

 
The West African craton (WAC) is composed of three Archaean and Palaeoproterozoic metamorphic and magmatic shields separated by two cratonic sedimentary basins (Fig. 1). The WAC components include: to the south the Man shield, to which the smaller Kayes and Kenieba inliers can be associated; to the north the Reguibat shield; and to the extreme north, the Anti-Atlas belt. In between are, in the centre, the huge Taoudeni basin and to the north the Tindouf basin. The Man and Reguibat shields comprise Archaean nuclei to the west (Feybesse & Milési 1994; Potrel et al. 1998; Key et al.). In the Man shield, a large part of the WAC consists of the Palaeoproterozoic Birimian continent (Abouchami et al. 1990; Liégeois et al. 1991; Boher et al. 1992; Soumaila et al.). The Reguibat shield contains Palaeoproterozoic assemblages in the eastern part as well as Archaean components that include kimberlites (Kahoui et al.). The Anti-Atlas belt comprises only a Palaeoproterozoic basement (Thomas et al. 2004). The WAC terranes were affected by the Eburnian orogeny, at around 2 Ga. During the Mesoproterozoic, no event or rocks are known in the WAC. This extremely quiet period between 1.7 Ga and 1 Ga allowed this large area to become a craton by acquiring a thick lithosphere (Black & Liégeois 1993). The palaeoposition of the WAC during the Mesoproterozoic is not known. By contrast to the Mesoproterozoic, the Neoproterozoic was an important period of evolution for the West African craton. At the beginning of the Neoproterozoic, the WAC was affected by several extensional events associated with continental breakup. The most important event was the formation of the Gourma aulacogen in Mali (Moussine-Pouchkine & Bertrand-Sarfati 1978) but also the deposition of passive margin sediments on its northern boundary in the Anti-Atlas (Bouougri & Saquaque 2004) and to the SE in the Voltas basin (Ako & Wellman 1985; Nédélec et al. 2007). The formation of the huge Taoudeni cratonic subcircular basin also began during that period (Bronner et al. 1980).

View larger version (82K): [in this window] [in a new window]   Fig. 1. Main geological units in West Africa, from Fabre (2005) and Liégeois et al. (2005). The positions and shapes of the Peri-Gondwanan terranes are based on Nance et al.

; Gürsu & Gonçuolu

	The boundaries of the West African craton

Top Abstract The West African craton The boundaries of the... The case study of... Conclusions References

 
The Pan-African orogeny induced a partial remobilization of the WAC boundaries, inducing a metacratonic evolution. At the end of the Neoproterozoic, the West African craton belonged to the subducting plate, implying that it was never an active margin above a subduction oceanic plate (Black et al. 1979; Hefferan et al. 2000; Ennih & Liégeois 2001; Villeneuve), with craton-ward directed fold-and-thrust structures (Jahn et al. 2001; Caby et al.) also forming outboard of the craton (Attoh & Brown). The West African craton acted as a rigid indenter during the Pan-African orogeny in a similar way as India is currently indenting the Asian continent (Black et al. 1979). The thick cratonic continental lithosphere was partly affected by transpression and transtension tectonic episodes, intrusion of granitoids and extrusion of volcanic sequences, such as in the Anti-Atlas (Ouarzazate Supergroup). High temperature/low pressure grade metamorphism occurred with large movements of fluids causing mineralization in several areas (Inglis et al. 2004; Kolb et al.; Belkabir et al.). Although partially buried by younger deposits, rock exposures contain excellent preservation of the early Neoproterozoic events such as the passive margin sediments (Bouougri & Saquaque 2004), the early thrust oceanic terranes (Samson et al. 2004; Bousquet et al.) and Pan-African magmatism (Touil et al.; Ezzouhairi et al.). The Pan-African orogeny generated large sediment influx, for example outside the WAC, within the Peri-Gondwanan terranes whose sedimentary sequences are marked by the WAC-typical signature, a bimodal set of zircon ages at c. 1.8–2.2 Ga and at 0.55–0.75 Ga (Nance et al.). The metacratonic evolution affected some parts of the WAC and peri-Gondwanan terranes such as the c. 2 Ga Icartian gneisses in Brittany (Calvez & Vidal 1978; Samson & D'Lemos 1998). A better knowledge of the boundaries of the West African craton is, therefore, of importance for the study of peri-Gondwanan terranes in Europe and in America.

During the latest Neoproterozoic and early Palaeozoic, the western WAC boundaries were first subjected to a major extensional event producing sedimentary and volcanic sequences (Alvaro et al.; Pouclet et al.) and the drifting of some of the peri-Gondwanan terranes (Nance et al.). Thick Phanerozoic sedimentary sequences were deposited afterwards up to Devonian times (Baidder et al.; Ouanaimani & Lazreq). The Late Palaeozoic Variscan orogeny moderately affected the WAC by generating thick-skin tectonics in the basement and thin-skin tectonics in the sedimentary cover rocks (Caritg et al. 2004; Burkhard et al. 2006; Baidder et al.; Dabo et al.; Soulaimani & Burkhard). In the Mesozoic, the western and southern WAC boundaries were subjected to the Atlantic rifting and Jurassic dolerite intrusions and massive Central Atlantic magmatic province (CAMP) basalt flows (Marzoli et al. 1999; Deckart et al. 2005; Verati et al. 2005). The Jurassic was also marked by the development of rifts on the eastern side (e.g. Gao rift) and on the northern side (e.g. Atlas belt), contemporaneously with the development of the Central Atlantic Ocean and the Western Mediterranean Sea (Laville et al. 2004; Guiraud et al. 2005). Finally, the Cenozoic era was marked by the convergence of the African and European continents, generating the High Atlas range, the uplift of the Anti-Atlas (Malusa et al. 2007) and the Cenozoic volcanism in West Africa (Berger et al.) and the Hoggar (Liégeois et al. 2005). The northern metacratonic boundary of the WAC is currently uplifted, forming the Anti-Atlas Mountains.

The boundaries of the WAC, metacratonized during the Pan-African orogeny, have been rejuvenated periodically. The rigid WAC metacraton has been affected by the reactivation of lithospheric faults that facilitate hydrothermal mineralization (Pelleter et al. 2007). This is the main characteristic of the metacratonic areas (Liégeois et al. 2003, 2005): being rigid but affected by faults of lithospheric scale, they constitute areas subjected to reactivation, including intraplate reactivations (Azzouni-Sekkal et al. 2003; Liégeois et al. 2005), making them areas likely to be rich in mineralizations.

	The case study of the remobilization of the Eburnian basement in the Anti-Atlas belt

Top Abstract The West African craton The boundaries of the... The case study of... Conclusions References

 
Our recent study on the Eburnian basement of the Anti-Atlas, in the Zenaga inlier (Figs 2 and 3) has revealed extensive REE mobility in Eburnian granites during the Pan-African orogeny. REE mobility is attributed to Pan-African transcurrent tectonism and associated Neoproterozoic Ouarzazate Supergroup volcanism.

View larger version (123K): [in this window] [in a new window]   Fig. 2. (a) Geological map of the Anti-Atlas (Morocco), from Thomas et al. (2004) and Gasquet et al. (2008). The rectangle outlines the area in Figure 1B. (b) Satellite photograph of the Anti-Atlas (Orthorectified Landsat Thematic Mapper Mosaics as compressed colour imagery in MrSIDTM file format from Lizardtech).

View larger version (98K): [in this window] [in a new window]   Fig. 3. (a) Satellite photograph of the Palaeoproterozoic Zenaga inlier (Orthorectified Landsat Thematic Mapper Mosaics as compressed colour imagery in MrSIDTM file format from Lizardtech). The inlier is noted ‘Zenaga’ on Figure 2a and 2b. (b) Sketch geological map of the Zenaga inlier (from Ennih & Liégeois 2001). The scales of the satellite image and of the geological map are the same.

The Anti-Atlas belt (Fig. 2) is separated in two parts by the Anti-Atlas major fault (AAMF) – long considered as the northern limit (e.g. Hefferan et al. 2000) of the WAC because it is marked by ophiolitic remnants, including that of Bou Azzer, and because Eburnian outcrops are not known north of it in the Saghro Mountains. For various geological but also rheological and isotopic reasons, Ennih & Liégeois (2001) proposed that the actual northern boundary of the WAC is the South Atlas fault that borders the High Atlas mountain range to the south (Fig. 2) of the AAMF. According to Ennih & Liégeois (2001), the South Atlas Fault marks the edge of the deepening of the WAC basement under Neoproterozoic volcano-sedimentary series. Here we will focus on the Zenaga inlier, which consists of Eburnian basement rocks and is located just to the south of the AAMF, for deciphering the Pan-African effects on the WAC northern boundary basement.

The Zenaga inlier

The Zenaga inlier is a depression of about 500 km² containing mainly Palaeoproterozoic gneisses and granitoids unconformably overlain by the late Neoproterozoic Ouarzazate volcanic Supergroup or by the Cambrian Tata Group (Fig. 3). Within the inlier, Neoproterozoic rocks also consist of passive margin sediments (Taghdout Group), pre-Pan-African doleritic dykes and sills and a late Pan-African alkaline ring-complex. Along the AAMF, to the west and to the east, remnants of the Bou Azzer–Sirwa oceanic terrane are present. A summary of the geology of the area helps to understand the metacratonic evolution of the Zenaga basement.

The Zenaga Palaeoproterozoic metamorphic rocks include medium to high-grade amphibolite facies grey gneisses, biotite-rich schists, garnet±sillimanite paragneisses, calc-silicate rocks, migmatites and rare amphibolites. The gneissic layering and the migmatitic leucosomes are deformed by isoclinal ductile folds, whose axes have variable plunge. This basement represents a high-grade metamorphic supracrustal series. These schists have not been dated but inherited zircons at c. 2170 Ma within the c. 2035 Ma cross-cutting granitoids could be attributed to the Zenaga schists (Thomas et al. 2002).

The Zenaga Palaeoproterozoic granitoids are represented by the Azguemerzi mesocratic granodiorite, and the Aït Daoui, Assersa, Tamarouft and Tazenakht granites. The Zenaga plutons show quartzo-feldspathic layers separated by biotite and garnet layers, locally associated with gneisses and anatectic products. They contain rare metasedimentary xenoliths and no mafic microgranular enclaves (MME). The presence of aluminous minerals (biotite, garnet, muscovite), the association with migmatitic rocks, the absence of MME suggest that the Zenaga granitoids originated by the partial melting of crustal rocks. The granodiorite and the granites have been dated at 2037±7 Ma, 2037±9 Ma and 2032±5 Ma (U–Pb zircon ages, Thomas et al. 2002). These dates give a minimum age for the gneisses and schists.

The Zenaga granitoid basement is unconformably covered by the Taghdout sedimentary Group, also known as the Tizi n-Taghatine Group (Thomas et al. 2004). The Taghdout Group displays brittle tectonic faults folds associated with a south-verging thrust event. Portions of this unit have been metamorphosed to greenschist facies. The Taghdout Group contains well-preserved sedimentary features, such as ripple marks, desiccation cracks or oblique stratification (Bouougri & Saquaque 2004). The Taghdout Group is a 2 km-thick succession deposited during three stages of an extensional event (Bouougri & Saquaque 2004): (1) a shallow-water and gently dipping mixed siliciclastic–carbonate ramp facing north and attached to braided alluvial plain in the south, indicating a relatively stable margin; (2) tholeiitic sills and dykes of the Ifzwane Group that cut the sedimentary sequence and the basement of the Zenaga inlier (particularly to the NW; Fig. 3a); (3) deepening of the margin marked by turbidites.

Although they are not directly in contact with the Zenaga basement, remnants of the Bou Azzer and Sirwa oceanic island arc complex occur along the AAMF (Fig. 2). This complex comprises ophiolitic sequences in which plagiogranites have been dated at 761±2 Ma and 762±2 Ma at Taswirine (Sirwa area; U–Pb zircon; Samson et al. 2004) and a tonalitic migmatite at 743±14 Ma at Iriri (Sirwa area; U–Pb zircon; Thomas et al. 2002). The zircon rims of the latter gave an age of 663±13 Ma, interpreted as the age of the metamorphism that accompanied the island arc accretion towards the craton (Thomas et al. 2002). In Bou Azzer, juvenile metagabbros (752±2 Ma), augen granite gneiss (753±2 Ma) and leucogranites (705±3 Ma; 701±2 Ma) are linked to this 750–700 Ma event but in a way still to be deciphered (D'Lemos et al. 2006).

The Zenaga basement is overthrust by the Tamwirine rhyolitic unit, attributed to the Bou Salda Group, which has been dated at 605±9 Ma (U–Pb zircon, Thomas et al. 2002). The Tamwirine rhyolites are unconformably overlain by the Ouarzazate Group. Rhyolites and granitoids of the Ouarzazate Group have been dated between 581±11 Ma and 543±9 Ma (Gasquet et al. 2005). Within the Zenaga inlier, the Sidi El Houssein alkaline granitic ring-complex (579±7 Ma; U–Pb zircon; Thomas et al. 2002) is contemporaneous with the Ouarzazate Group.

Structurally, the c. 2 Ga Eburnian deformation was high-grade and north–south to NE–SW orientated (Ennih et al. 2001). The Pan-African deformation occurred under greenschist conditions and is mostly NW–SE oriented along the AAMF corridor (Ennih et al. 2001). The Anti-Atlas was later deformed during the Late Palaeozoic Variscan orogeny, which was responsible for the generation of domes and for major décollements between the Palaeoproterozoic basement and the Neoproterozoic/Phanerozoic cover. Variscan deformation produced spectacular disharmonic folds in the Ouarzazate and Tata Groups and listric extensional faults within the basement (Faik et al. 2001; Burkhard et al. 2006), reactivating faults generated at the end of the Pan-African orogeny (Soulaimani et al. 2004). Those extensional structures were again reactivated during the Cenozoic Alpine orogeny generating the current relief of the Anti-Atlas with its Precambrian inliers and Cenozoic volcanism (Berger et al.). Within the Palaeoproterozoic basement, attributing a structure to the Pan-African, Variscan or to the Alpine events is not easy because of the strong rheological contrast between the rigid basement and the softer sedimentary cover, inducing partition of the deformation. Even the rare thrust faults in the Zenaga basement that are generally interpreted as Pan-African in age could be Variscan or even Alpine in age (Thomas et al. 2002). However, it seems that the Variscan and Alpine events never induced thermal effects above 300 °C, based on the 580–525 Ma biotite-whole-rock mineral Rb–Sr dates obtained (Thomas et al. 2002).

The deformed Eburnian Zenaga granitoids are strongly peraluminous in character.

Analytical techniques

Whole-rock major and trace elements. Major elements have been measured by X-ray fluorescence (Université Catholique de Louvain) and the trace elements by ICP-MS (VG PQ2+, Royal Museum for Central Africa). For trace elements, the result of the alkaline fusion (0.3 g of sample+0.9 g of lithium metaborate at 1000 °C during one hour) has been dissolved in 5% HNO₃. The calibrations were set using both synthetic solution (mixture of the considered elements at 2, 5 and 10 ppb) and international rock standards (BHVO-1, W1, GA, ACE). For all these elements, the precision varies from 5 to 10% (for details, see Navez 1995). Results are given in Table 1 (major elements) and Table 2 (trace elements).

The Azguemerzi porphyritic biotite-granite is a coarse-grained, zoned and mesocratic rock. The primary minerals are mainly plagioclase and mega-alkali feldspar which are resorbed and largely sericitized. The mega-alkali feldspar occurs as coarse perthitic orthoclase megacrysts and medium grained microcline. Myrmekitic intergrowths are uncommon, but when present, pervasive and large orthoclase megacrysts are occasionally surrounded by a composite mantle of plagioclase and quartz. The quartz is typically interstitial and forms late stage inclusions. The main mafic magmatic silicates are biotite, garnet and the epidote, which is included in or associated with biotite (most of the epidote is secondary). Accessory minerals include anhedral, rounded zoned zircon, euhedral apatite, titanite, and ilmenite. Secondary minerals are sericite, chlorite and most of the epidote. The Azguemerzi pluton shows quartzo-feldspathic layers separated by biotite layers, locally associated with gneisses and anatectites in the Assersa and in Tizi-n-Taguergoust valleys. The Azguemerzi pluton displays a magmatic fabric which evolves locally to a true foliation, which probably explains why it was sometimes regarded as porphyritic gneiss. It contains xenolithic micaschists and gneisses of metasedimentary nature which could represent the source of these rocks but are most probably xenoliths from the country-rocks.

The Assersa, Aït Daoui and Tamarouft are granodiorite and monzogranite plutons that show the same mineralogical assemblage as the Azguemerzi pluton without biotite, conferring their leucocratic character. The Tazenakht granite in the northern part of Zenaga is a heterogeneous coarse-grained rock. It consists of abundant euhedral alkali feldspar phenocrysts, xenomorphic crystals of quartz, subhedral sericitized polycrystalline plagioclase, twisted biotite and sometimes twisted muscovite. Decimetre-size pegmatitic pockets are associated with acidic pegmatitic and aplopegmatitic dykes. Accessory minerals are mainly oxides; rare corundum has been observed. This granite was generally deformed in a solid state; it has a planar structure formed with mega-alkali feldspar sometimes fractured, twisted and kinked muscovite and biotite with heterogeneous levels corresponding to mylonitic rocks. Towards the south, the mega-alkali feldspars are deformed within a very intense foliation near the contact of the Azguemerzi granite. These characters reflect an intense and heterogeneous deformation, locally transforming the Tazenakht granite into orthogneiss, porphyroblastic mylonites and phyllonitic layers.

A main characteristic of the Zenaga granitoids is the abundant presence of peraluminous minerals with abundant muscovite and almandine-rich garnet (Alm_71-89 Pyr_3-14 Sps_2-12), except the Tazenakht granite which does not bear garnet but is particularly rich in muscovite and locally contains corundum.

Major elements

The studied Palaeoproterozoic granitoids (location in Fig. 3b) are all mainly felsic (SiO₂>68%) except the Azguemerzi granodiorite, which is intermediate in composition (most samples are in the range 61–70% SiO₂). They have variable compositions in alkalis and straddle the boundaries defined for the alkalic, alkali-calcic, calc-alkalic and calcic series (Fig. 4a) suggesting a heterogeneous source or some alkali mobility. They are always strongly peraluminous (Fig. 4b) and yield an alumina saturation index (ASI) decreasing with silica, which points to a peraluminous melt crystallizing aluminous minerals. The Al₂O₃ activity in the melts compared with the ASI (Patiño-Douce 1992) shows that the Zenaga granitoids are chemically comparable to other garnet bearing granitoids (Fig. 4c) pointing to a heterogeneous peraluminous source rather than to alkali mobility.

View larger version (11K): [in this window] [in a new window]   Fig. 4. (a) SiO2 vs. Na2O+K2O-CaO (MALI index; Frost et al. 2001). (b) Alumina saturation index (ASI) = Al2O3/(Na2O+K2O+CaO) in molar proportions vs. SiO2 showing the strong peraluminous character of the Palaeoproterozoic Zenaga granitoids. (c) A* (=ASI *(Na2O+K2O)) vs. ASI (Patiño-Douce 1992); boxes for the three kind of peraluminous granitoids are based to the analyses compiled by Patiño-Douce (1992).

REE and Nd isotopes as markers of the Pan-African metacratonic evolution of the Zenaga Eburnian basement

The rare earth element (REE) patterns of the five plutons studied display very varied shapes considering that all rocks are within the 64–75% SiO₂ range. The Azguemerzi granodiorites display normal REE patterns for granodiorite, except the large variability in HREE abundance that can be attributed to variable garnet control (Fig. 5a). The Tazenakht granite (Fig. 5b) has one sample (TA8) very low in REE and with no Eu negative anomaly, which is very different from the two other samples; the Eu abundance of sample TA4 is similar to that of sample TZK3 but its other REE are much higher, suggesting a possible beginning of tetrad effect (enrichment in REE due to F-rich fluids; Bau 1996; Veksler et al. 2005). The three samples from the Tamarouft pluton (Fig. 5c) display low abundance of REE, low REE fractionation and variable Eu anomaly. Sample TGR48 displays an REE pattern that could be magmatic; the two other samples show unusual spectra: sample TGR43 is richer in normalized HREE than in LREE and sample AN39 has a strong positive Eu anomaly and also higher normalized HREE than LREE. The three samples from the Ait Daoui pluton (Fig. 5d) have similar HREE but very dissimilar LREE. Sample AD28 has a normal magmatic pattern whereas sample AD26 suffered a strong loss in LREE and sample AD24 has nearly no Eu anomaly and a weak LREE/HREE fractionation. The three samples from the Assersa granite (Fig. 5e) display from La to Eu spectra similar to seagull patterns but a strong depletion in HREE. This suggests the influence of F-rich fluids (for the tetrad effect) coupled to the destabilization of a HREE-rich mineral such as garnet, present in this granite.

View larger version (18K): [in this window] [in a new window]   Fig. 5. Rare Earth Elements normalized to chondrites (Boynton 1984) for the studied Zenaga granitoids, showing the F-rich fluids influence on some of the samples. (a) Azguemerzi granodiorite; (b) Tazenakht granite; (c) Tamarouft granite; (d) Ait Daoui granite; (e) Assersa granite.

Initial ¹⁴³Nd/¹⁴⁴Nd values relative to bulk Earth (_Nd) are highly variable in the studied plutons: they vary from –36 to +16. This variation is observed within all the plutons studied, although the most extreme values belong to the Tamarouft and Ait Daoui plutons (Table 3).

When looking closely at the evolution of the _Nd through time for the different plutons (Fig. 6), several observations emerge. The Azguemerzi granodiorite (Fig. 6a) displays the expected evolution for a magmatic rock: similar slope (proportional to the ¹⁴⁷Sm/¹⁴⁴Nd ratio), grouped _Nd at 2035 Ma (U–Pb zircon crystallization age), and a progressively larger variability of _Nd while time is elapsing (common _Nd at 2035 Ma, slight difference in ¹⁴⁷Sm/¹⁴⁴Nd ratios between samples inducing progressive difference in the produced radiogenic ¹⁴³Nd). The Tazenakht granite (Fig. 6b) shows the opposite behaviour: _Nd are distinct at 2035 Ma and become more and more similar with time. The Tamarouft and Ait Daoui plutons (Fig. 6c, 6d) display crossed patterns: _Nd are very different at 0 Ma and 2035 Ma, having a common value during the Neoproterozoic. Finally, the Assersa pluton (Fig. 6e) shows parallel evolution, the difference in _Nd of the three samples remaining nearly constant through time.

View larger version (28K): [in this window] [in a new window]   Fig. 6. Nd isotopes (shown as Nd=[(143Nd/144Nd)sample-(143Nd/144Nd)Bulk-Earth]/(143Nd/144Nd)Bulk-Earth, both sample and Bulk Earth Nd isotopic compositions being calculated at the time considered. Full lines are the evolution calculated with the measured 147Sm/144Nd ratio of the sample (single stage evolution). The dashed lines from 600 Ma (age of the Pan-African perturbation) to 2035 Ma (age of the granite crystallization; U–Pb zircon, Thomas et al. 2002) are the evolutions calculated with the 147Sm/144Nd ratio of unaltered samples during the Pan-African orogeny, from the same pluton (two-stage evolution). See text for more explanation. (a) Azguemerzi granodiorite; (b) Tazenakht granite; (c) Tamarouft granite; (d) Ait Daoui granite; (e) Assersa granite; (f) All plutons together with measured magmatic 147Sm/144Nd ratios (single stage evolution) or adopted 147Sm/144Nd ratios (two-stage evolution).

The unaltered Azguemerzi samples (Fig. 5a) can be taken as reference for the magmatic signature of the Eburnian Zenaga plutons: their _Nd at 2035 Ma vary between +0.3 and +1.9. In the Ait Daoui pluton, the AD28 sample has a magmatic REE pattern (Fig. 5d) and gives a _Nd of +1.8, within the Azguemerzi range. This sample can, therefore, be considered as having a REE magmatic signature. Its ¹⁴⁷Sm/¹⁴⁴Nd ratio can be used from 600 to 2035 Ma for the two other samples of the Ait Daoui pluton: with this two stage evolution, sample AD24 get a _Nd at 2035 Ma of +2.5 and sample AD26 a _Nd of +2.3 (Fig. 6d), very close to the Azguemerzi range. In a similar way, sample TGR48 from the Tamarouft pluton can be considered as having a magmatic REE signature (_Nd at 2035 Ma=–0.8) and when using its ¹⁴⁷Sm/¹⁴⁴Nd ratio from 600 to 2035 Ma (Fig. 6e), one of the other samples get an Azguemerzi-like Nd values (sample TGR43, _Nd at 2035 Ma=+0.8) and the other sample (AN39) get a lower Nd of –3.3 but however much more magmatic compatible that its single stage _Nd of +16.4 (Fig. 6d).

The Tazenakht TZK23 sample, which shows a classical magmatic REE pattern (Fig. 5b), has a _Nd at 2035 Ma of +3.1. With the TZK23 ¹⁴⁷Sm/¹⁴⁴Nd ratio, the two other samples provide _Nd at 2035 Ma of +1.6 and +4.1 (Fig. 6b). Finally, if the samples from the Assersa pluton have lost a part of their HREE content (Fig. 5e), a feature likely to be linked to the fact that the garnet in this pluton is altered to chlorite and epidote, their LREE values appear to be less affected. Their _Nd at 2035 Ma are +0.8 (Asra 9), –0.36 (Asra11) and –5.03 (As112). The first two are within the range of the Azguemerzi pluton. Sample As112 has a lower _Nd but this sample is REE-poor and with a Sm concentration of 1.4 ppm (against the measured value of 1.6 ppm), this sample would have a _Nd at 2035 Ma of +0.38 (Fig. 6e).

These results show that, when using magmatic ¹⁴⁷Sm/¹⁴⁴Nd ratios deduced from pristine samples for all samples from 600 Ma to 2035 Ma, the obtained _Nd at 2035 Ma are quite homogeneous, varying from –0.8 to +2.5 for most samples, with one sample at –3.3 and two samples at +3.1 and +4.1 (Fig. 6f); with the recalculated values, the mean _Nd at 2035 Ma for the Zenaga plutons is remarkably determined at +1.1±0.9. This indicates a mainly juvenile source (_Nd of the depleted mantle at 2035 Ma=+5.5), which is also indicated by the T_DM model ages of the samples having ¹⁴⁷Sm/¹⁴⁴Nd<0.15 (measured or recalculated) whose mean is 2159±61 Ma. The calculation for the other samples would have implied a three-stage evolution and would have given similar model ages. They denote that major REE fractionation existed in the source, which, coupled with the strongly peraluminous character of the Zenaga granitoids, is consistent with a metasedimentary juvenile continental crustal source combining metasedimentary formations and mafic rocks melted at depth. More details about the nature of the Eburnian orogeny in the Anti-Atlas require further constraints on the Zenaga metamorphic basement whose age, according to the inherited zircons dated in the granitoids, would be around 2.17 Ga (Thomas et al. 2002).

Sr isotopes have been largely modified by the Pan-African event: at 2035 Ma, most of the ⁸⁷Sr/⁸⁶Sr initial ratios are much lower than 0.7. The alignment (with a poor MSWD of 190) determined by the 15 samples gives an ‘age’ of 1467±310 Ma (initial ⁸⁷Sr/⁸⁶Sr=0.711±0.078). This ‘age’ is the result of the interplay of the Eburnian age of the granitoids and the major Pan-African effect.

	Conclusions

Top Abstract The West African craton The boundaries of the... The case study of... Conclusions References

 
The chemical and isotopic data for the Zenaga Palaeoproterozoic magmatism indicate that these peraluminous granitoids originated from the partial melting of a juvenile, largely metasedimentary, crustal source but that the Pan-African orogeny has strongly reactivated some plutons or parts of plutons, including REE. The likely cause was the circulation of F-rich fluids circulation linked to the extrusion of the voluminous Ouarzazate Group and the emplacement of associated plutons (c. 580 Ma) along reactivated faults and shear zones. The reactivation event occurred during important vertical movements (the thickness of the Ouarzazate Group varies from 0 to more than 2500 m) but without major crustal or lithospheric thickening. This is demonstrated by the low-grade character of the Pan-African metamorphism (greenschist facies) and by the excellent preservation of the c. 800 Ma passive margin sediments and the early c. 750–700 Ma ophiolitic complex. This is considered to be typical of a metacratonic evolution: the cratonic boundary was dissected by faults and invaded by magmas but preserved most of its rigidity and primary features. Such a metacratonic reactivation was favourable for fluid circulation, in this case able to mobilize rare-earth elements. Such fluid movements were also an excellent vector for element concentration and genesis of mineralizations, for which the Anti-Atlas is internationally renowned.

This book concerns an example of these areas of paramount importance that are the boundaries of craton that suffered partial reactivation, i.e. a metacratonic evolution. The West African craton is a particularly good example because: (1) it became a strong craton during the Mesoproterozoic, a period of 600 Ma that left no trace on the WAC; (2) all its boundaries intervened as indentors during the Pan-African orogeny leading to situations varying from nearly frontal collision to nearly entirely transcurrent dockings; (3) only its western and northern boundaries were included as foreland in the Variscan collision, allowing fruitful comparison between the eastern and the western WAC boundaries; (4) the WAC boundaries were major suppliers of lithospheric pieces or of sedimentary material for the Peri-Gondwanan terranes now located in Europe or in North America; (5) currently, the WAC boundaries are reactivated by the stress generated by the Africa-Europe convergence and constitutes a key area for studying intraplate deformation submitted to stress; (6) WAC boundaries, or at least a part of them, are known for their mined or potential mineral deposits. A better knowledge of the boundaries of the West African craton is a prerequisite for understanding all these processes. This is the aim of the papers constituting this Special Publication.

	Acknowledgments

 
This is a contribution to the IGCP485. We warmly thank the UNESCO and the IUGS for their financial support and encouragements during these five years. We are happy that this Special Publication comes out during the International Year of Planet Earth, an outstanding outcome from these two organizations. Organizing contacts and field meetings in the desert regions of Mauritania, Mali, Algeria and Morocco was always a challenge but also very stimulating. We would like to thank Khalidou Lô (Mauritania), Samba Sacko (deceased), Dramane Dembélé, Renaud Caby (Mali), Khadidja Ouzegane, Abla Azzouni-Sekkal (Algeria), Ezzoura Errami Hassan Admou, Hassan Ouanaimani, and Abdelilah Fekkak (Morocco) for their personal support and the support of organizations in their country. We are also grateful to the Universities of Algiers (USTHB) and El Jadida, the Office Mauritanien de Recherches Géologiques, the Direction Nationale de la Géologie et des Mines de Bamako, the Algerian COMENA as well as the Ministries of Mines and Energy of Mauritania, Mali and Algeria. We warmly thank Bernard Bonin for judicious remarks on key points of the paper and Kevin Hefferan for detailed and helpful remarks and corrections. Both have suggested significant improvements to this article. Finally, we would like to thank the Publication team of the Geological Society for their support and patience.

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